2007 X5 Off Road

I'm considering getting a new 2007 X5 4.8, but I have a few questions.

I need an SAV b/c I have two dogs, and do a lot of mountain and road cycling. I really love the 20 in wheels (and am not too crazy about the 19's that come with the Sport package). How are the 20's off road? I don't do any real off-roading, just the occasional gravel road or grass parking lot when going to mountain bike. Can I use the 20's offroad at all? Or will I need to change them out every time I want to go offroad?

I live in Texas, so snow is not much of an issue, but I do take occassional cross country trips to the north. I'll likely get a 2nd set of 18" wheels and tires. Should I get snow tires or all-season? How are snow tires on the highway at 80+ mph? How long do they last? Any recommendation for tires that are good in the snow, light offroading, and decent on-road performance?

Lastly, does anyone know how much Discount Tire (or any other place) charges to swap a wheel set? I'm thinking I might change the wheels just for road trips to the north or on mtn bike trips when I know I'll be offroad.

Thanks.